The invention relates to a soft switching topological circuit, more particularly to a soft switching topological circuit in a boost or buck converter circuit and a bridge circuit.
The prior art working procedures of a ZVT-BOOST circuit are shown as in FIGS. 2A-2F. FIG. 2A shows the waveform of the gate driving signal voltage Vgs2 of the auxiliary MOSFET 106; FIG. 2B shows the waveform of the gate driving signal voltage Vgs1 of a main MOSFET 103; FIG. 2C shows the waveform of the current Ilr in resonant inductor 105; FIG. 2D shows the waveform of current IDmain of main ultrafast recovery diode 107; FIG. 2E shows the waveform of the current IDaux of auxiliary ultrafast recovery diode 108; FIG. 2F shows the waveform of the voltage Vdsmain between the source and the drain of main MOSFET 103. It can be seen from the above drawings:
When t=to, auxiliary MOSFET 106 is turned on, since the current flowing through an inductor, can not change abruptly, so, when auxiliary MOSFET 106 is turned on, the current in resonant inductor 105 increases gradually from an initial value, therefore the current flowing through main ultrafast recovery diode 107 decreases, and gradually to zero, thereby a soft turn-off of main ultrafast recovery diode 107 is realized by means of resonant inductor 105;
It can be seen from FIG. 2D, at the moment t=t1, the forward current of main ultrafast recovery diode 107 reduces to zero smoothly, thereby realizing a soft turn-off of main ultrafast recovery diode 107;
After the soft turn-off of main ultrafast recovery diode 107, resonant inductor 105 resonates with resonant capacitor 104, as shown in FIG. 2F, at the moment t=t2, when the voltage on resonant capacitor 104 resonates to zero, i.e., the voltage Vdmain between the drain and source of main MOSFET 103 is also zero, the parasitic diode of main MOSFET starts turn-on and freewheel.
During the freewheeling period of the parasitic diode of main MOSFET 103, at the moment t=t3, main MOSFET is turned on while auxiliary MOSFET 106 is turned off, thus, realizing a zero voltage turn on of main MOSFET 103, at this moment, the stored energy in resonant inductor 105 is fed into output filter capacitor 109 through auxiliary ultrafast recovery diode 108, since the voltage between the drain and source of auxiliary MOSFET 106 is limited by the voltage Vo on output filter capacitor 109 through auxiliary ultrafast recovery diode 108, thereby also realizing a voltage clamping of auxiliary MOSFET 106 when it is turned off;
As shown in FIG. 2E, at the moment t=t4, the stored energy in resonant inductor 105 is completely released, i.e. the current flowing through auxiliary ultrafast recovery diode 108 is reduced smoothly to zero, and a soft turn-off of auxiliary ultrafast recovery diode 108 is realized,
At the moment t=t5, main MOSFET 103 is turned off, resonant capacitor connected in parallel to main MOSFET 103 accomplishes a zero voltage turn-off of main MOSFET 103 as shown in FIG. 2F; along with the rise of the voltage Vdsmain between the drain and source of main MOSFET 103, voltage Vdsaux between the drain and source of auxiliary MOSFET 106 will also rise due to the resonance of resonant inductor 105 and the output parasitic capacitor of auxiliary MOSFET 106, and the current flowing through resonant inductor 105 also rises resonantly, as shown in FIG. 2C;
At the moment t=t6, when the voltage Vdsaux between the drain and source of auxiliary MOSFET 106 equals the voltage on output filter capacitor 109, i.e. equals to the voltage Vo on load resistor 110, the current in resonant inductor 105 will flow to the output filter capacitor 109 through auxiliary ultrafast recovery diode 108, while at this moment main ultrafast recovery diode 107 is turned on, thus the voltage drop withstood on resonant inductor 105 is zero, it can be seen according to V=Lrxc2x7di/dt=0, the current flowing through resonant inductor 105 remains unchanged until auxiliary MOSFET 106 is turned on, therefore, at the moment t=t7, when auxiliary MOSFET 106 is turned on again periodically, it is a non-zero current turn-on.
The converter circuit has been disclosed in China Patent Application CN 95190525.2. The circuit diagrams and the working procedures are shown in FIGS. 1 and 2. When the circuit is at the moment t=6, and the voltage Vdsaux between the drain and source terminal of an auxiliary MOSFET 106 equals the voltage on an output filter capacitor 109, i.e. the voltage Vo on load resistor 110, the current of resonant inductor 105 flows to output filter capacitor 109 through auxiliary ultrafast recovery diode 108, but at this time, main ultrafast recovery diode 107 is turned on, therefore, the voltage drop of resonant inductor 105 is zero, it can be seen from V=Lrxc2x7di/dt=0 that before auxiliary MOSFET 106 is turned on, the current flowing through the resonant inductor 105 remains unchanged so, therefore at the moment t=t7, when auxiliary MOSFET 106 is turned on again periodically, it is a nonzero current turn-on.
Due to the above reason, the turn-on of auxiliary MOSFET 106 at the moment t=to is a non-zero current turn-on, thereby resulting in a fact that the turn-off of auxiliary ultrafast recovery diode 108 at t=to is a hard turn-off, so the turn-on loss of auxiliary MOSFET 106 and the turn-off loss of the corresponding auxiliary ultrafast recovery diode 108 are relatively large.
The invention gives out an improved ZVT power converter circuit, through which the drawbacks of the above-mentioned invention can be overcome, thus realizing a zero-current turn-on for the auxiliary MOSFET and a soft turn-off for auxiliary ultrafast recovery diode.
A basic principle on the invention is to utilize the resonance of a resonant inductor and a resonant capacitor after the auxiliary switch is turned on to realize a zero-voltage turn-on for the main switch. What is more important is that the energy feed device has no energy feed-out when the auxiliary switch is turned on, thereby achieving a zero current turn-on for the auxiliary switch, and the circuit running efficiency is raised.
The invention includes the following circuit, which comprising: a main switch, an auxiliary switch, a freewheel diode in parallel with the main switch, a resonant capacitor, a current source, a resonant inductor, a main diode, an energy-feed device and a voltage source. In which the resonant capacitor is connected to the main switch in parallel, the main and auxiliary switches are turned on and off periodically, at the same time when the auxiliary switch is turned off, the main switch is turned on simultaneously, but the auxiliary switch is not turned on until the main switch is turned off for a period of time. In the boost converter circuit, said current source and said auxiliary switch form a loop, wherein the cathode of said main diode is connected to the positive electrode of said voltage source to form a serial branch, which is connected in parallel to said main switch; in the buck converter circuit, said voltage source, main switch and main diode form a loop, wherein the negative electrode of said voltage source is connected to the anode of the main diode, the current source is connected in parallel to the serial branch formed by the main diode and said resonant inductor. In these two converters, the resonant inductor is inserted between the current source and the connecting point of the main diode and said main switch, said auxiliary switch is connected in parallel to the serial branch formed by said resonant inductor and said main switch, said energy-feed device feeds out the residual energy of the resonant inductor when said auxiliary switch is turned off, and meanwhile feeds out the energy of the current source.
The basic principle on the invention is to utilize the resonance of a resonant inductor and a resonant capacitor after the auxiliary switch is turned on to realize a zero-voltage turn-on for the main switch. What is more important is that the energy feed device has no energy feed-out when the auxiliary switch is turned on, thereby achieving a zero current turn-on for the auxiliary switch, and the circuit running efficiency is raised.
The task of the invention is solved through the following circuit, which comprising: a main switch, an auxiliary switch, a freewheel diode in parallel with the main switch, a resonant capacitor, a current source, a resonant inductor, a main diode, an energy-feed device and a voltage source. In which the resonant capacitor is connected to the main switch in parallel, the main and auxiliary switches are turned on and off periodically, at the same time when the auxiliary switch is turned off, the main switch is turned on simultaneously, but the auxiliary switch is not turned on until the main switch is turned off for a period of time. In the boost converter circuit, said current source and said auxiliary switch form a loop, wherein the cathode of said main diode is connected to the positive electrode of said voltage source to form a serial branch, which is connected in parallel to said main switch; in the buck converter circuit, said voltage source, main switch and main diode form a loop, wherein the negative electrode of said voltage source is connected to the anode of the main diode, the current source is connected in parallel to the serial branch formed by the main diode and said resonant inductor. In these two converters, the resonant inductor is inserted between the current source and the connecting point of the main diode and said main switch, said auxiliary switch is connected in parallel to the serial branch formed by said resonant inductor and said main switch, said energy-feed device feeds out the residual energy of the resonant inductor when said auxiliary switch is turned off, and meanwhile feeds out the energy of the current source.
The energy-feed device of the invention can be a diode, i.e. an auxiliary diode, and the auxiliary diode is connected in parallel to a serial branch formed by the resonant inductor and the main diode.
The above resonant capacitor can be a parasitic capacitor of said main switching device, said freewheel diode may be an inverse-parallel diode or a parasitic diode of the main switching device.
The circuit of the invention ensures that the auxiliary diode is definitely cut off before the auxiliary switch is turned on, thus ensuring a zero current turn-on of the auxiliary switch, and also avoiding the hard turn-off of the auxiliary diode, and raising the circuit efficiency. However, the auxiliary switch is still a hard turn-off.
To solve the hard turn-off problems of auxiliary switch, a lossless snubber diode and a lossless snubber capacitor can be added in the above-mentioned circuit. Wherein the lossless snubber diode is connected in series to said auxiliary diode, and the serial branch thus formed is connected in parallel to a serial branch formed by said resonant inductor and main diode, and said lossless snubber capacitor is a cross the connecting point of said lossless snubber diode and said auxiliary diode and the connecting point of said resonant inductor and the main diode.
The circuit realizes a zero current turn-on and zero voltage turn-off of the auxiliary switch, and further increases the efficiency of the circuit.